Electronic Theatre Controls, Inc.

Eric G Rasmussen

John P Thurk

Eric W Suomi

Kevin Burke

A current sensing transformer having a low impedance primary winding is connected in series with a filter and load circuit supplied with output current by a high frequency switching PWM power supply such as a sinewave dimmer. Sensed output current is supplied to a microprocessor based programmable controller. The controller performs a routine including a family of overcurrent tests. Tests of the family are satisfied by the presence of different output current values for different time durations. At least one of the tests is enabled only at low requested output voltages or dimming levels. In response to satisfaction of any of the tests, a series power switching stage of the power supply is rendered nonconductive to shut down the power supply.

Low voltage overcurrent protection for solid state switching system

Current sensing resistors are connected directly to output terminals of MOSFETs forming the series switching stage of a PWM sinewave dimmer. An analog sensed current signal is coupled through an EMI resistant current link to comparators that provide an overcurrent indication in response to an overcurrent condition. A logic circuit overrides PWM control signals from a programmable controller, and operates drivers to render the series switching stage nonconductive and to render a clamp switching stage conductive until after the end of the current PWM duty cycle power on segment. The logic circuit sends an overcurrent signal through an optocoupler to an input of the programmable controller. The input of the controller is polled at a clock frequency for overcurrent signals and if the count exceeds a limit in any half cycle, or a maximum sum in sixteen half cycles of the AC supply, the series switching stage is rendered nonconductive and the clamp switching stage is rendered conductive pending a reset signal.

Overcurrent protection for solid state switching system

The solid state switching section of a high frequency PWM switching power supply may encounter problems in accurately regulating output power when the requested output power is above a threshold level, the threshold level typically being a high level approaching one hundred percent of the power of the sinusoidal AC power source. Below the threshold level, the power supply is operated in a full PWM mode to regulate the output power level. Above the threshold level, the power supply is operated in a full on, continuously conducting mode in a central part of the half cycles of the AC supply. Outside of this central portion, the power supply is operated in the in the PWM mode, using a duty cycle within the capability of the solid state switching section. The output power level is regulated by varying the duration of the central full on portion of the half cycle. A similar control method is used for reducing heat generated by the solid state switching section of the power supply.

PWM switching power supply control methods

An input line filter separates an ac sinusoidal power source from a power switching stage having a series switching section and a clamp switching section. Both the series switching section and a clamp switching section include complementary pairs of high power and high speed MOSFETs. The series switching section is connected in series between the input line filter and the inductor of an output load filter between the power switching stage and a load. The clamp switching section is connected to shunt the inductor and load. Synchronous operation is achieved by alternately operating the series and clamp switching sections to conductive conditions at a high pulse width modulation frequency. The duty cycle is selected to provide a sinusoidal output waveform with a desired amplitude. A microprocessor based programmable controller provides control signals for alternate conduction of the series and clamp switching sections. A dead time interval stored in memory in the controller separates the control signals for reliable and efficient operation. The dead time interval may be constant, or may be varied in accordance with load power variations.

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Waunakee, WI, United States of America

Eric G Rasmussen